This invention relates generally to borehole formation evaluation instrumentation. More particularly, this invention relates to electromagnetic propagation tools using small loop magnetic dipole antennas for use primarily in oil and gas well drilling applications.
Borehole formation evaluation tools are known which measure phase and/or amplitude of electromagnetic waves to determine an electrical property (such as resistivity or permitivity) of a section of a borehole. Typically, the existing tools used for this application are composed of one or more transmitting antennas spaced from one or more pairs of receiving antennas. An electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes by the receiving antennas. In a resistivity measuring tool, magnetic dipole antennas are employed which operate in the mf and lower hf spectrum. In contrast, permitivity tools utilize magnetic dipole antennas in the VHF or UHF ranges.
In a known resistivity sensor of the type hereinabove discussed, the resistivity sensor measures both phase difference and amplitude ratio to provide two resistivities with different depths of investigation. A signal received in a first receiving antenna is shifted in phase and its amplitude will be less than the signal received in a second receiving antenna. Resistivities are then derived from both the phase difference and the amplitude ratio of the received signals. This differential measurement is primarily responsive to the formation opposite the receiving antennas and is less sensitive to the borehole and/or variations in the transmitted signal as in prior art sensing devices.
While well suited for its intended purposes the transmitter and receiver loop antennas suffer from several problems and disadvantages. These loop antennas generally comprise coils wrapped around the drill string. The coils are recessed in circumferentially cut grooves about the drill string. This is an attempt to protect the antenna coils from the harsh downhole environment encountered by measurement-while-drilling (MWD) tools. Even with the grooves these antenna coils are often damaged during the drilling process. This has been attributed to the size of the coils required for generating and detecting the electromagnetic signals (i.e., waves).
The circumferential grooves tend to structurely weaken the drill string. This has two affects, firstly, the number of loop antennas in a tool is restricted by the weakening of the tool and secondly, any attempts to employ smaller diameter tools is restricted due to the depth of the circumferential grooves.